SYNOPSIS.It is shown that by making certain simplifying assumptions quantitative results can be obtained from a geometrical vector representation of phase contrast. Formulae for the intensity and contrast in the general case, and in a number of special cases, including central dark‐ground illumination, are derived, and are shown to be identical with those obtained by more elaborate mathematical procedures. The present paper is mainly devoted to a detailed discussion of some aspects of positive phase contrast. The appearances of (a) pure phase objects, and (b) mixed phase and amplitude objects are considered for (1) a non‐absorbing 90° phase plate, (2) 90° phase plates of any absorption, and (3) non‐absorbing phase plates in which the phase of the direct light is advanced by any angle Ψ. The bearing of some of the results on the selection of the optimum type of phase plate for a given purpose is d